1. Field of the Invention
The present invention relates to electrophotographic photoconductors for liquid development, which are used for image forming according to a liquid developing method that uses a developing solution containing toner particles in a liquid, image-forming apparatuses comprising the photoconductors and image-forming methods.
2. Description of the Related Art
Photoconductors for use in an electrophotographic type have been roughly categorized into two types, that is, an inorganic photoconductor and organic photoconductor. Here, the electrophotographic type refers to an image-forming process, so called Carlson Process. Specifically, in general, photoconductor is initially charged, for example, by a corona discharge in the dark, is exposed imagewise, the charge only at the exposed portion is scattered selectively to obtain an electrostatic latent image, this latent image portion is developed using a toner comprising a colorant, such as a dye and a pigment, and a polymer material, and the latent image is visualized to thereby form an image.
The developing method in the electrophotography according to the Carlson Process is mainly classified into a dry developing method and a wet developing method or liquid developing method. At the present, the image-forming apparatus using the dry developing method is widely applied, for example a copying machine, and a printer, and is commonly used. On the other hand, the image-forming apparatus utilizing the wet developing method has been developed and commercialized from the old times. However, the image-forming apparatus using the dry developing method accounts for most of the market.
However, with respect to the image-forming apparatus utilizing the wet developing method, generally, the toner is dispersed in a liquid and the toner particles can be rendered extremely fine, thus the obtained image can possess an extremely high image quality. Therefore, in recent years, accompanying with a market expansion of a full color printer to which a high image quality is required, the image-forming apparatus utilizing the wet developing method is attracting the attention again and the development thereof is progressed.
As mentioned above, since the image-forming apparatus utilizing the wet developing method uses a developing solution in which the toner particles are dispersed in a liquid, the whole part or a part of the used photoconductor is immersed in the above-noted liquid developing solution. As for the liquid (carrier solvent) used for the developing solution, for example, an aliphatic hydrocarbon solvent, which is called Isopar (trade name; manufactured by Exxon Chemicals), a silicone oil, or the like is mainly used. As for a photoconductor, an inorganic photoconductor, such as selenium and amorphous silicon, by which a photoconductor component is not eluted into a carrier solvent is generally used.
On the other hand, an organic photoconductor is advantageous, in comparison with an inorganic photoconductor, in available wavelength region for exposure, film-forming properties, flexibility, transparency of film, mass-productivity, toxicity and cost, thus the photoconductor in which an organic material is used has been actively developed and is put into practice.
This kind of organic photoconductor is mainly classified into a laminated layer photoconductor comprising a charge-generating layer having a charge generating function and a charge-transporting layer having a charge transporting function; and a single-layer photoconductor comprising a single layer having both the charge generating function and the charge transporting function. The former has a configuration in which a charge-generating layer and a charge-transporting layer are disposed on the support in this order, and is applied mainly to an image forming apparatus according to a negative charging system from a restriction with respect to an organic material. The former is excellent in photosensitive properties and durability, thus is widely put into practice. On the other hand, the latter has a configuration in which a single photosensitive layer is disposed on the support and is applied from the viewpoint of easiness in principle to obtain a high image resolution, mainly to an image-forming apparatus according to a positive charging system.
Therefore, an inorganic photoconductor, such as selenium and amorphous silicon, which is generally used in an image-forming apparatus utilizing the above-mentioned wet developing method, is usually used by positively charging the photoconductor, thus when a conventional inorganic photoconductor is replaced by an organic photoconductor, it is advantageous that a single-layer photoconductor can be used in the same image-forming apparatus according to the positively charging system as that in which an inorganic photoconductor is previously used.
When a commonly used organic photoconductor is employed in an image-forming apparatus utilizing a wet developing method, as mentioned above, the whole or a part of used photoconductor is immersed in a liquid developing solution (carrier solvent), resulting in the cracking of the photoconductor due to the contact with the carrier solvent, the crystallization of the compound having a low molecular mass, such as a charge-transporting material and/or an acceptor compound, or the elusion of these compounds into the developing solution. This brings about a remarkable deterioration not only mechanically but also electrically, accordingly a satisfactory image cannot be obtained.
Thus, an organic photoconductor has been proposed in which an overcoat layer (surface protective layer) containing a thermosetting resin such as a silicone resin, an epoxy resin and a melamine resin, which is insoluble in a liquid developing solution, is disposed on the surface of the organic photoconductor. However, by disposing the overcoat layer, new problems arise that the sensitivity of the photoconductor is extremely impaired, and besides production cost becomes high.
Alternatively, as a method in which the overcoat layer is not disposed, Japanese Patent Application Laid-Open (JP-A) Nos. 2002-116560, 2002-131943, 2002-351101, 2002-40677, 2000-214610 and 2003-5391 propose a single-layer photoconductor for use in the wet developing method. By using a specific binder resin, the single-layer photoconductor has high resistance to a carrier solvent used in the wet developing method, a charge-transporting material does not elute into this solvent, and the photoconductor has a practical sensitivity.
In these proposals, a binder resin having a relatively high polarity is used, thereby improving the resistance of the photoconductor to a carrier solvent having a low polarity. Therefore, the elution of the charge-transporting material is substantially inevitable, causing a problem that such a photoconductor is not durable to a long-term usage.
Further, JP-A No. 2000-63456 proposes a copolymer between a chemical structure block having a charge transporting function and a chemical structure block of the binder resin; and a single-layer photoconductor using the same. In this proposition, it is described that when this copolymer is used in an electrophotographic photoconductor according to a wet developing method, the elution of a compound having a low molecular mass of the photoconductor caused by a liquid developing solution can be prevented. This single-layer photoconductor has a certain level of sensitivity; however, not so high as to satisfy the requirement of the market fully.
Further, JP-A No. 2003-57856 proposes a copolymer between a chemical structure block having a charge transporting function and a chemical structure block of the binder resin; and a single-layer photoconductor using the above-mentioned copolymer.
However, since the above-proposed copolymer comprises a chemical structure block having a charge transporting function in an amount of 5 mol % to 30 mol %, the charge transporting does not exhibit satisfactory charge transport performance. When only the copolymer assumes the charge transporting function, satisfactory sensitivity of the photoconductor cannot be obtained. Therefore, for obtaining satisfactory sensitivity, as shown in Examples, addition of a charge-transporting material having a low molecular mass is required. When such a photoconductor is used as an electrophotographic photoconductor according to a wet developing method, there is a problem that the elution of the charge-transporting material having a low molecular mass caused by a liquid developing solution is inevitable and the photoconductor is not durable to a long-term usage.